FIG. 1 shows a conventional current mirror circuit. The conventional current mirror circuit includes transistors M1, M2 and M3. The transistor M1 carries the reference current Ii while the second transistor M2 carries the output current Io. The transistors M1 and M2 have gates coupled together in order to ensure they have identical gate-source voltages so that the reference current Ii is mirrored to the output current Io, i.e., the current Io flowing in the channel of the transistor M2 is proportional to the reference current Ii.
FIG. 2 shows another conventional current mirror circuit which is often referred to as a cascode current mirror. The cascoded transistors are arranged in series to the first transistor M1 and the second transistor M2. Similar to the circuit shown in FIG. 1, identical gate-source voltages on the transistors result in the reference current Ii being mirrored to the output current Io. However, when the drain voltage is higher than a threshold voltage, the current will flow from the drain to the substrate directly rather than through the channel. This is the so-called “hot carrier” effect. The substrate current resulting from “hot carrier” effect under high drain-to-source voltages leads to current mismatch between the input and output current.
In order to solve the current mismatch issue, circuits shown in FIG. 3 and FIG. 4 are proposed. An NMOS transistor M6 and operational amplifier 302 are included in the circuit of FIG. 3. However, the substrate current of the transistor M6 still causes the mismatch between the reference current Ii and the output current Io. In the circuit of FIG. 4, two NMOS transistors M7 and M8 are included so that the drain voltages of the transistors M1 and M2 are kept close to each other. However, the current leakage in the transistors M7 and M8 still results in current mismatch. Furthermore, the need for the bias voltages Va and Vb complicates the circuit design.